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3  -- Copyright (c) Jeremy Siek, Lie-Quan Lee, and Andrew Lumsdaine 2000
4  --
5  -- Distributed under the Boost Software License, Version 1.0.
6  -- (See accompanying file LICENSE_1_0.txt or copy at
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9<Head>
10<Title>Boost Graph Library: Dijkstra Visitor</Title>
11<BODY BGCOLOR="#ffffff" LINK="#0000ee" TEXT="#000000" VLINK="#551a8b" 
12        ALINK="#ff0000"> 
13<IMG SRC="../../../boost.png" 
14     ALT="C++ Boost" width="277" height="86"> 
15
16<BR Clear>
17
18<H1><img src="figs/python.gif" alt="(Python)"/>Dijkstra Visitor Concept</H1>
19
20This concept defines the visitor interface for <a
21href="./dijkstra_shortest_paths.html"><tt>dijkstra_shortest_paths()</tt></a>
22and related algorithms. The user can create a class that matches this
23interface, and then pass objects of the class into
24<tt>dijkstra_shortest_paths()</tt> to augment the actions taken during
25the search.
26
27<h3>Refinement of</h3>
28
29<a href="../../utility/CopyConstructible.html">Copy Constructible</a>
30(copying a visitor should be a lightweight operation).
31
32
33<h3>Notation</h3>
34
35<Table>
36<TR>
37<TD><tt>V</tt></TD>
38<TD>A type that is a model of Dijkstra Visitor.</TD>
39</TR>
40
41<TR>
42<TD><tt>vis</tt></TD>
43<TD>An object of type <tt>V</tt>.</TD>
44</TR>
45
46<TR>
47<TD><tt>G</tt></TD>
48<TD>A type that is a model of Graph.</TD>
49</TR>
50
51<TR>
52<TD><tt>g</tt></TD>
53<TD>An object of type <tt>G</tt>.</TD>
54</TR>
55
56<TR>
57<TD><tt>e</tt></TD>
58<TD>An object of type <tt>boost::graph_traits&lt;G&gt;::edge_descriptor</tt>.</TD>
59</TR>
60
61<TR>
62<TD><tt>s,u,v</tt></TD>
63<TD>An object of type <tt>boost::graph_traits&lt;G&gt;::vertex_descriptor</tt>.</TD>
64</TR>
65
66<TR>
67<TD><tt>DistanceMap</tt></TD>
68<TD>A type that is a model of <a href="../../property_map/ReadWritePropertyMap.html">Read/Write Property Map</a>.</TD>
69</TR>
70
71<TR>
72<TD><tt>d</tt></TD>
73<TD>An object of type <tt>DistanceMap</tt>.</TD>
74</TR>
75
76<TR>
77<TD><tt>WeightMap</tt></TD>
78<TD>A type that is a model of <a href="../../property_map/ReadWritePropertyMap.html">Readable Property Map</a>.</TD>
79</TR>
80
81<TR>
82<TD><tt>w</tt></TD>
83<TD>An object of type <tt>DistanceMap</tt>.</TD>
84</TR>
85
86</table>
87
88<h3>Associated Types</h3>
89
90none
91<p>
92
93<h3>Valid Expressions</h3>
94
95<table border>
96<tr>
97<th>Name</th><th>Expression</th><th>Return Type</th><th>Description</th>
98</tr>
99
100<tr>
101<td>Initialize Vertex</td>
102<td><tt>vis.initialize_vertex(u, g)</tt></td>
103<td><tt>void</tt></td>
104<td>
105This is invoked one each vertex of the graph when it is initialized.
106</td>
107</tr>
108
109<tr>
110<td>Examine Vertex</td>
111<td><tt>vis.examine_vertex(u, g)</tt></td>
112<td><tt>void</tt></td>
113<td>
114This is invoked on a vertex as it is popped from the queue. This
115happens immediately before <tt>examine_edge()</tt> is invoked
116on each of the out-edges of vertex <tt>u</tt>.
117</td>
118</tr>
119
120<tr>
121<td>Examine Edge</td>
122<td><tt>vis.examine_edge(e, g)</tt></td>
123<td><tt>void</tt></td>
124<td>
125This is invoked on every out-edge of each vertex after it is discovered.
126</td>
127</tr>
128
129<tr>
130<td>Discover Vertex</td>
131<td><tt>vis.discover_vertex(u, g)</tt></td>
132<td><tt>void</tt></td>
133<td>
134This is invoked when a vertex is encountered for the first time.
135</td>
136</tr>
137
138<tr>
139<td>Edge Relaxed</td>
140<td><tt>vis.edge_relaxed(e, g)</tt></td>
141<td><tt>void</tt></td>
142<td>
143Upon examination, if the following condition holds then the edge
144is relaxed (its distance is reduced), and this method is invoked.<br>
145<tt>
146tie(u,v) = incident(e, g);<br>
147D d_u = get(d, u), d_v = get(d, v);<br>
148W w_e = get(w, e);<br>
149assert(compare(combine(d_u, w_e), d_v));<br>
150</tt>
151</td>
152</tr>
153
154<tr>
155<td>Edge Not Relaxed</td>
156<td><tt>vis.edge_not_relaxed(e, g)</tt></td>
157<td><tt>void</tt></td>
158<td>
159Upon examination, if the edge is not relaxed (see above) then
160this method is invoked.
161</td>
162</tr>
163
164<tr>
165<td>Finish Vertex</td>
166<td><tt>vis.finish_vertex(u, g)</tt></td>
167<td><tt>void</tt></td>
168<td>
169This invoked on a vertex after all of its out edges have been added to the
170search tree and all of the adjacent vertices have been discovered
171(but before their out-edges have been examined).
172</td>
173</tr>
174
175</table>
176
177<h3>Models</h3>
178
179<ul>
180 <li><a href="./dijkstra_visitor.html"><tt>dijkstra_visitor</tt></a>
181</ul>
182
183<a name="python"></a>
184<h3>Python</h3>
185
186To implement a model of the <tt>DijkstraVisitor</tt> concept in Python,
187create a new class that derives from the <tt>DijkstraVisitor</tt> type of
188the graph, which will be
189named <tt><i>GraphType</i>.DijkstraVisitor</tt>. The events and syntax are
190the same as with visitors in C++. Here is an example for the
191Python <tt>bgl.Graph</tt> graph type:
192
193<pre>
194class count_tree_edges_dijkstra_visitor(bgl.Graph.DijkstraVisitor):
195  def __init__(self, name_map):
196    bgl.Graph.DijkstraVisitor.__init__(self)
197    self.name_map = name_map
198
199  def edge_relaxed(self, e, g):
200    (u, v) = (g.source(e), g.target(e))
201    print "Relaxed edge ",
202    print self.name_map[u],
203    print " -> ",
204    print self.name_map[v]
205</pre>
206
207<br>
208<HR>
209<TABLE>
210<TR valign=top>
211<TD nowrap>Copyright &copy 2000-2001</TD><TD>
212<A HREF="../../../people/jeremy_siek.htm">Jeremy Siek</A>,
213Indiana University (<A
214HREF="mailto:jsiek@osl.iu.edu">jsiek@osl.iu.edu</A>)<br>
215<A HREF="../../../people/liequan_lee.htm">Lie-Quan Lee</A>, Indiana University (<A HREF="mailto:llee@cs.indiana.edu">llee@cs.indiana.edu</A>)<br>
216<A HREF=http://www.osl.iu.edu/~lums>Andrew Lumsdaine</A>,
217Indiana University (<A
218HREF="mailto:lums@osl.iu.edu">lums@osl.iu.edu</A>)
219</TD></TR></TABLE>
220
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